1. Field of the Invention
The present invention generally relates to a color image forming apparatus and a color image forming method and, in particular, to a tandem electrophotographic color image forming apparatus including separate image forming units for color components and a color image correcting method.
2. Description of the Related Art
There are tandem color image forming apparatuses such as printers or copying machines that include as many electrophotographic color image forming units as the number of available color components and transfer toner images of the color components one after another onto printing media, using the image forming units. The image forming units for each color include a developing machine and a photosensitive drum. It is known that there are multiple factors that can cause misalignments (called registration errors) of images of the color components in a tandem color image forming apparatus.
The factors include nonuniformity and misalignment of lenses of a deflecting scanning unit including an optical system such as polygon mirrors and fθ lenses and misalignment of attachment of the deflecting scanning unit to the image forming apparatus. These misalignments prevent a scan line from becoming parallel with the axis of rotation of a photosensitive drum and cause an inclination or skew of the scan line. If the degree of inclination and skew of scan lines (hereinafter referred to as the profile or shape of a scanning line) differs among the colors, registration errors occur.
The characteristics of profiles vary among image forming apparatuses, that is, recording engines. In addition, deflecting scanning units for different colors have different profile characteristics. FIGS. 6A to 6D show exemplary profiles. In FIGS. 6A to 6D, the abscissa represents positions in the main scanning direction of an image forming apparatus. The straight line 600 extending in the main scanning direction in FIG. 6A represents the ideal characteristic (profile) of a scan line without a skew. Curves 601 and 602 (FIG. 6B), curve 603 (FIG. 6C), and curve 604 (FIG. 6D) represent the profiles of scan lines of different colors, cyan (hereinafter abbreviated as C), magenta (M), yellow (Y), and black (K), respectively. The ordinate represents the amount of shift in the sub scanning direction with respect to the ideal characteristic. As can be seen from the figures, the curves of the profiles vary from color to color. When electrostatic latent images are formed on photosensitive drums of the image forming units for the individual colors, the differences in profile appear as registration errors of image data of the colors.
A method for handling registration errors is described in Japanese Patent Laid-Open No. 2002-116394. In the method, the magnitude of a skew of a scan line is measured with an optical sensor in the fabrication process of a deflecting scanning device, the skew of the scan line is adjusted by mechanically rotating a lens, and then the lens is fixed with an adhesive.
Japanese Patent Laid-Open No. 2003-241131 describes a method in which, in a process for attaching a deflecting scanning device to a color image forming apparatus, an inclination of a scan line is measured with an optical sensor and the deflecting scanning device is tilted to adjust the inclination of the scan line before the deflecting scanning device is attached to the color image forming apparatus.
Japanese Patent Laid-Open No. 2004-170755 describes a method in which the magnitudes of an inclination and skew of a scan line are measured with an optical sensor, bitmap image data is corrected to cancel the inclination and skew, and the corrected image is formed. In particular, a misalignment of an actual scan line with respect to a straight line on the surface of a photosensitive drum that is parallel to the axis of rotation of the photosensitive drum, that is, an ideal scan line, is canceled by shifting image data in the opposite direction by the amount of the misalignment. The method does not require a mechanical adjustment member and an adjustment process because the method corrects image data. Accordingly, the method can reduce the size of the color image forming apparatus and can handle registration errors less expensively than the methods described in Japanese Patent Laid-Open No. 2002-116394 and No. 2003-241131. The electrical correction of registration errors is divided into pixel-wise correction and sub-pixel-level correction. In pixel-wise correction, pixels are shifted (offset) pixel by pixel in the sub scanning direction in accordance with the amounts of an inclination and skew to be corrected as shown in FIGS. 15A to 15C. In the following description, the position at which an offset is made is referred to as a scan line changing point and the process for offsetting is referred to as a scan line changing process. Points P1 to P5 in FIG. 15A are scan line changing points.
In FIG. 15A, the profile 1501 of the scan line is to be corrected. While the profile 1501 may be represented by a sequence of coordinate values of pixels on the scan line, for example, the profile 1501 is represented by an approximating curve divided according to areas in FIG. 15A. A scan line changing point is a position in the main scanning direction at which a misalignment of one pixel occurs with respect to the sub scanning direction when the profile is scanned in the main scanning direction. In FIG. 15A, points P1 to P5 are such points. The dots subsequent to a scan line changing point are shifted by one line in the direction opposite to the sub scanning direction in the profile. This operation is performed for each line. FIG. 15B shows an example of image data resulting from shifting at each scan line changing point in the sub scanning direction. The shaded portions 1511 in FIG. 15B represent one line before the scan line changing process, that is, one line in original image data. As a result of the scan line changing process, each line is shifted in the direction in which the misalignment of the profile with respect to the sub scanning direction is cancelled. FIG. 15C shows an example of image data obtained as a result of the shifting. The shaded portions represent one line before the correction. During formation of an image, corrected image data is formed line by line. For example, normal image formation is performed in the sequence of lines 1521, 1522, . . . , and so on. As a result, the shaded portions constituting one line in the image data before the correction are formed on an ideal scan line on which the line is to be formed. However, sub-pixel misalignments in the sub scanning direction remain because the scan line changing process is performed on a pixel-by-pixel basis.
Therefore, sub-pixel misalignments that cannot be corrected by the scan line changing process are corrected by adjusting the tone values of bitmap image data using pixels around scan line changing points in the sub scanning direction. That is, when the profile characteristic indicates an upward inclination with respect to the scanning direction, bitmap image data before tone correction is corrected so that a pixel sequence is inclined in the direction (downward in this example) opposite to the inclination indicated by the profile. In order to approximate the image data to ideal corrected image data, tone correction is applied to pixels near the scan line changing points to smooth the steps at the scan line changing points. The smoothing can be accomplished by using the pulse width or intensity of laser, for example. The tone correction applied for smoothing after the scan line changing process is hereinafter called interpolation process.
Depending on the nature of an image, the interpolation process of image data may improve or impair the image quality of the image. For example, the visibility of information of an image such as an image containing a pattern of a repetitive geometric figure (hereinafter referred to as a pattern image), characters, or fine lines that can be drawn using office document processing software can be improved by the interpolation process for smoothing. In contrast, the quality of a continuous tone image produced by a screening process may be degraded when the interpolation process is applied to pixels near scan line changing points, because the interpolation process can cause inconsistencies in density at the portions near the scan line changing points. For example, when a line growth screen is used, the interpolation process changes the thickness of lines constituting the screen at the scan line changing points, giving an appearance of varying densities from a macroscopic viewpoint. Further, when the interpolation process is applied to an ad-on image such as a background pattern consisting of background areas and hidden image areas described in Japanese Patent Laid-Open No. 2004-223854, the effect of the ad-on image can be impaired, therefore the interpolation process is not appropriate for such images.
In this way, when the interpolation process is uniformly applied to an entire image without taking into consideration the characteristics of image data, the quality of the image can be degraded. Therefore, determination must be made as to whether the interpolation process should be applied to image data of interest, on the basis of attributes or features of the image data. An invention has been proposed that detects features of an image before halftoning or pixel-wise correction is applied to the image, and then halftoning or an exceptional process are applied to the image according to the result of the detection (see Japanese Patent Laid-Open No. 2006-297633). The exceptional processing includes sub-pixel interpolation.
However, the invention disclosed in Japanese Patent Laid-Open No. 2006-297633 is intended to process image data before halftoning or pixel-wise correction. That is, features of input continuous tone image data are detected and then halftoning or an exceptional process is applied to the image data according to the result of the detection.
On the other hand, quantization of image data is essential to reduce consumption of limited processing resources and speed up processing because quantization compresses the amount of data and reduces the load of data processing. Even if input data is continuous tone image data, it is desirable to first quantize the image data to reduce the amount of data and to reduce the load of the subsequent processing.
However, in the related art documents given above, no mention is made of a method of detecting features of halftoned (quantized) image data before the subsequent processing. Therefore, the image quality of a printout of received facsimile image data and image data converted into binary data by an application program can be degraded.
Furthermore, the related-art techniques perform a scan line changing process to correct misalignments of a scan line and then smooth the line by the interpolation process. However, image data after the scan line changing process has lost the continuity of one line and therefore the related-art techniques cannot detect features from the image data itself.